Head-up displays ("HUDs") for use in aircraft are well known. Generally speaking, an aircraft head-up display is an optical display apparatus for projecting symbology of vital flight parameters into the pilot's field of view while he is sitting in the normal "head-up" position looking out a forward portion of the windscreen in a conventional manner. A head-up display integrates outside visual cues with internally generated images conveying aircraft preformance data to assist the pilot in a variety of conditions. Such a system can provide useful and important information about the operating condition of the aircraft without requiring the pilot to divert his attention from outside the aircraft, refocus on an instrument panel, and then redirect his attention to the outside world.
A wide variety of flight parameter symbology can be provided, depending upon the requirements of the particular installation. In commercial aircraft, the head-up display can provide information pertaining to the aircraft's heading, altitude, airspeed, attitude, and landing environment. In military aircraft, the head-up display can advantageously be used for targeting purposes. An indicia of the aiming point of the aircraft's weaponry, such as a cross-hair, is provided on the head-up display. When the image of the cross-hair is superimposed on the desired target, the pilot knows it is appropriate to actuate the weapon. lf the sights are properly calibrated, the round should strike the target.
A typical head-up display may consist of several components, for example, a control panel, a symbol generator, a video drive unit, and a pilot's display unit. The symbol generator is a small digital computer which uses its own programmed logic to convert inputs from the aircraft sensors into the CRT drive instructions. These instructions are provided to the video drive unit which converts the input signals into the CRT drive voltages. These drive voltages generate the symbols projected to the pilot by the pilot's display unit.
The pilot's display unit ("PDU") comprises a housing to which are mounted a CRT and an optical module containing a combiner glass and collimating lens. The optical module reflects an image generated by the CRT to the eye of the pilot, while also allowing light from outside the aircraft to pass through the optical module to the pilot's eye. Thus, the head-up display combines images from the outside world showing what is actually in front of the aircraft, together with the images created by the CRT. Since the pilot's display unit must be interposed into the pilot's normal line of vision, the positioning of the PDU within the cockpit is critical.
In helicopters, as opposed to fixed wing fighter or bomber aircraft, monocular head-up displays have become popular. As the name implies, a monocular HUD is one in which the CRT projects the flight parameter symbology into only one eye of the pilot. The other eye is available for normal observation outside the aircraft.
The pilot's display unit of a monocular head-up display normally attaches to a fixed overhead mounting and is suspended directly in front of the pilot's eye when the head-up display is in use. To provide a large field of view from an optic of minimal size, the display is preferably located only about four inches from the pilot's eye in its operational or "boresight" position. Because of the proximity of the hardware to the pilot's head, when the pilot is performing other operations within the aircraft that do not require the use of the head-up display, it is important that the display be easily removable from its operative position directly in front of the pilot's face such that it does not restrict his movement or obscure his vision.
Thus, there is a need to provide a mounting apparatus for the pilot's display unit of a head-up display system which permits easy removal of the PDU from its operational position, in which it is interposed in the pilot's line of sight, to a stowage position in which it is out of the way.
A further practical restraint for monocular head-up displays is to make sure the pilot's display unit can be quickly and easily swung from its stowage position to its operational position by the pilot without unduly impairing his operation of the aircraft. With particular reference to military aircraft, it will be appreciated that flying under combat conditions is an extremely stressful undertaking. When a target is spotted, necessitating that the display unit be lowered to its operational position, the pilot must be able to do so very quickly and in a manner which does not interfere with the safe operation of the aircraft.
Those familiar with helicopters will appreciate that they are equipped with two hand controls for operating the aircraft. The pilot's right hand operates the cyclic control, which controls the critical parameters of pitch and roll of the helicopter. On the left is the collective lever, controlling the pitch of the rotor blades. While the pilot can safely momentarily release his grip of the collective lever without endangering the aircraft, it is strongly preferred to maintain the pilot's right hand on the cyclic control at all times, and especially at low altitudes.
Accordingly, there is a further need to provide a mounting apparatus for the pilot's display unit of a head-up display which permits quick and easy movement of the PDU between its stowage and operational positions using only the pilot's left hand, while the pilot's right hand remains on the controls.
A further important aspect of the mounting apparatus for the pilot's display unit of a monocular head-up display in an aircraft concerns the necessity for protecting the pilot's safety in the event of sudden deceleration of the aircraft. This condition would be encountered, for example, in the event of a aircraft crash. If an aircraft is engaged in a survivable crash, the sudden, almost instantaneous, deceleration of the aircraft would cause the pilot's head to be thrown violently forward. With the pilot's display unit interposed in the pilot's line of vision only four inches from the pilot's face, provision must be made to permit the PDU to break away from its boresight position upon impact by the pilot's helmet.
Further complicating this problem is the fact that when the pilot's display unit, having thus been broken away from its operational position, suddenly encounters the forwardmost extremity of its range of movement, it will tend to rebound violently rearwardly. In view of the forces encountered in typical crash environments, a display unit thus hurtling rearwardly towards the pilot's head and impacting with the pilot's facial or jawbones can cause serious, or even fatal, injury.
Efforts have been made to provide a mounting apparatus which permits the pilot's display unit to be removed to a stowage position out of the pilot's line of vision, and which affords protection to the pilot in the event of a crash. One such apparatus is disclosed in U.S. Pat. No. 4,188,090 to Ellis. Ellis discloses a mounting apparatus for the pilot's display unit of a monocular head-up display whereby the PDU is pivotable about an axis of rotation. In a first embodiment, the axis of rotation is overhead and transverse to the longitudinal axis of the aircraft. In an alternative embodiment, the single axis of rotation is inclined with respect to the aircraft's orthagonal axes at such an angle that the pivoting movement of the PDU has a major component to the left or right of the pilot's line of sight. The display unit is maintained in its operational position by a compliant member which yields if the PDU is impacted by a sudden accidental forward movement of the pilot's helmet, permitting the PDU to break away during a crash. This design further contemplates that the pilot's display unit can be moved to a stowage position, either by pivoting the PDU forwardly until it swings out of the pilot's line of sight, or by pivoting the PDU rearwardly to stow it flush against the overhead.
The design disclosed in Ellis suffers a number of disadvantages. First, such a design cannot be adapted to provide a suitable stowage position for the PDU in a large number of aircraft. Initial forward rotation of the display unit moves the lower edge of the unit along an arcuate path having a major horizontal component and only a minor vertical component. Thus, the PDU must be rotated a substantial distance forward in order to impart any appreciable vertical movement to the lower end of the display unit. In other words, in order to utilize a forward stowage position, the pilot's display unit must be swung a considerable distance forward before the lower end of the display unit is raised out of the pilot's line of vision. However, since the windscreens for most helicopters are positioned as close to the pilot as possible in order to afford maximum visibility in all directions, there often is insufficient space forward of the pilot to permit the display unit to be rotated forward far enough to be raised completely out of the pilot's line of sight. Such is particularly the case with the widely-used Sikorsky S70 helicopter, making the mounting arrangement disclosed in Ellis inappropriate for retrofit installation into this popular helicopter.
The mounting arrangement disclosed in Ellis is also impractical for moving the pilot's display unit between its operational attitude and a rearward stowage position. In order to store the PDU rearwardly against the overhead, since the display unit in its operational position is only four inches from the pilot's eye, and since the first major component of motion as the PDU is swung rearwardly is towards the pilot's face, stowing the PDU rearwardly requires that the pilot duck to the side as the PDU is being raised and lowered between its stowage and operational positions. Such maneuvers are not only inconvenient but also impede the pilot's ability to control his aircraft.
A further disadvantage to the apparatus disclosed in Ellis is that, while the display unit will break away upon impact to swing forwardly away from the pilot, no provision is made to prevent the unit from rebounding rearwardly as it encounters the forward limit of its range of movement. As previously mentioned, even if the pilot is not seriously injured by his initial impact with the display unit, he can still sustain severe injury by the PDU rebounding violently toward the rear of the aircraft.
Thus, there is a need to provide a mounting apparatus for a pilot's display unit which breaks away from its operational position upon impact by a pilot's helmet, and which is restrained from rebounding rearwardly to prevent further injury to the pilot.
A further problem associated with the prior art head-up display concerns the difficulty associated with replacing the pilot's display unit in the event of a failure. As is well-known, the CRT mounted within the housing of the pilot's display unit has one of the highest failure rates of any component in the head-up display system. In order to replace a typical pilot's display unit of the design disclosed in Ellis, screws securing a cover plate must first be removed to provide access to the mounting bolts. The mounting bolts must then be removed to release the pilot's display unit from its mount. To install the replacement PDU, the steps must be performed in a reverse sequence. It will be appreciated that this procedure requires a variety of tools and, especially in light of the cramped quarters inside an aircraft cockpit, can take as long as a half hour to accomplish. It will be further appreciated that the numerous components and fasteners needed for mounting the prior-art pilot's display unit require precise machining and increased tolerance requirements in the construction of the mounting apparatus, thereby resulting in higher construction costs. Accordingly, there is a need to provide a quick-replacement mounting apparatus for the pilot's display unit of a head-up display system, wherein the display unit can be quickly and easily replaced with a minimum of tools.
There is a further need to provide a pilot's display unit mounting apparatus which reduces the tolerances required during manufacture, thereby providing less expensive construction.